Generally, pitch for matrix is used for the preparation of carbon materials, for example, by impregnating carbonaceous materials such as carbon fiber, porous carbonaceous material and the like with said pitch for matrix, followed by thermopolimerizing in an inert atmosphere.
Such pitch for matrix as mentioned above is desired to have a high flowability at the time when it is heated for impregnation and also desired to be carbonized in a high yield during the thermopolymerization thereof.
In this connection, there has heretofore been made an attempt to increase the yield of carbonization of pitch for matrix by air oxidation, such as air blow, of the starting pitch consisting essentially of an aromatic compound in its molten state. According to this method, however, there is involved such a problem that it is difficult to obtain uniform carbon materials having excellent characteristics, because the oxidation reaction of the starting pitch is slow in progress owing to insufficient contact of the pitch with oxygen and said oxidation reaction does not proceed uniformly.
A method is also known to pulverize the starting pitch or mold the starting pitch into a fiber form, followed by air oxidation thereof. In this method, however, involved is such a problem that when the carbonization yield of the starting pitch is increased by means of sufficient oxidation, the oxidized pitch does not melt so as to flow and, on the other hand, when the oxidation of said starting pitch is limited to such an extent that the pitch melts to flow, the carbonization yield of the starting pitch does not increase sufficiently.
Known also is a technique to obtain petroleum resin or phenol resin suitable for use in coating materials, adhesives, pressure-sensitive adhesives, casting materials, sealing materials and the like by oxidation polymerization of the starting aromatic compound in the presence of a crosslinker such as formaldehyde and an acid catalyst. And though it may be presumable that such petroleum resin or phenol resin as referred to above is used as a raw material for the preparation of carbon materials, it is difficult to obtain desired carbon materials in a high carbonization yield.
By the way, Japanese Patent L-O-P Publn. No. 139080/1988 discloses a process for producing graphite, which comprises treating carbonaceous mesophase or coke with nitric acid or a mixed acid of nitric acid and sulfuric acid, followed by heating at a temperature of at least 2400.degree. C. Further, Japanese Patent L-O-P Publn. No. 169339/1991 discloses a process for producing carbonaceous adsorbents having micro pores, which comprises treating a mesophase obtained by heat treatment of pitch at at least about 350.degree. C. or a coke with nitric acids or a mixed acid of nitric acid and sulfuric acid, and eluting insolubles out of the carbon material by the treatment with a solvent, followed by the heat treatment of the thus treated product at 300.degree.-1400.degree. C. The above-cited publications, however, fail to disclose a process for preparing pitch for matrix excellent in impregnating properties and the like.
Further, Japanese Patent Publn. No. 27302/1990 discloses a process for preparing pitch for carbon materials, which comprises subjecting powdered pitch or fibrous pitch to oxidizing treatment, and molding the oxidized pitch under a molding pressure of 100-500 kg/cm.sup.2 G into a desired form, followed by carbonization. In the process for preparing the pitch disclosed in this publication, however, the oxidation treatment employed, to which the powdered or fibrous pitch is subjected, is limited only to the vapor phase oxidation.
According to the process comprising the vapor phase oxidation of molten pitch as illustrated above, there was a limit to the use of the resulting oxidized pitch as a carbonaceous material, particularly as the pitch for matrix of carbon material, because no uniform pitch could be obtained owing to a slow oxidation reaction of the pitch, and when the vapor phase oxidation reaction temperature was elevated in order to accelerate the oxidation reaction of the pitch, a thermopolymerization of the pitch took place concurrently therewith, and hence the resulting oxidized pitch was found poor in flowability.
In the process for the vapor phase oxidation of pitch in its powdered or fibrous state, it is presumed that the oxidation reaction proceeds up to the interior of the powder or fiber, and the pitch comes to decrease in its flowability even when it is in a heated state, and hence it is considerably difficult to increase the carbonization yield of the pitch while keeping flowability of said pitch being oxidized.
Further, it is also known to obtain pitch for carbon material by liquid phase oxidation of a mesophase or coke. In this process, however, there is involved such a problem that this process is low in economical efficiency, because this liquid phase oxidation is prone to ununiformity, and for obtaining a uniform oxidized pitch, the liquid phase oxidation treatment for an extended period of time is required.
With the view of solving the above-mentioned problems, the present inventors prosecuted extensive researches and have accomplished eventually the present invention on the basis of their findings that a pitch having a low softening point and an optical isotropy is formed in advance into a fibrous form and the thus treated pitch may be nitrated in a short period of time even at a low reaction temperature by liquid phase nitration with a nitration reagent containing an acid catalyst and, as the result, the thus nitrated pitch is high in thermal reactivity, and hence the thermopolymerization reaction of the nitrated pitch can proceed with a low energy and, moreover, that because the nitrated pitch is thermopolymerized without evaporation of low molecular weight components at the time of thermopolymerization of the pitch, the carbonization yield of the pitch increases, and the nitrated pitch thus obtained is low in softening point and excellent in moldability and impregnability as matrix for various composite materials.